Duality

In a sense duality is just something related to the hyperplanes in separation theory. While duality used calculus and differentials to represent the tangent price lines, separation theory used inner product of vectors to represent the line. Varian's book is really looking more like a devolution from Takayama's, in terms of the use of mathematics, which was introduced into economics to clarify things better, and now people are going back to complicating them.
However this might be rather a necessary situation. Whenever a new existence comes in, nature has to integrate it into its system by enriching it in two directions, complicating and simplifying, so that it gets connection to everything else, and so that the connections are binded together under a single rule. Both simplicity and complexity are natural. Humans do best if they follow the natural way. And for the intellect as an existence in the world, both complexity and simplicity is needed for it to integrate into the world. In this sense, the popular Chinese system of the complicating Confucious together with the simplifying Buddhism, or rather Zenism, is quite a working combination.